Process control for flash concentrating solutions containing polyolefins

ABSTRACT

Apparatus and methods are provided for controlling liquid levels in different vessels, and the blending of different materials in the separation and purification of polymeric materials in solution, such as polyethylene in cyclohexane.

BACKGROUND OF THE INVENTION

The invention relates to apparatus and methods for controlling theconcentrations of solute in hydrocarbon solvent.

In the several methods of polymerizing aliphatic olefins to solidpolymers having high molecular weight, hydrocarbon solvents, such ascyclohexane, are frequently used to facilitate the polymerizationreaction and also the subsequent transfer and separation steps. See forexample, Hogan, et al., U.S. Pat. No. 2,825,721, Mar. 4, 1958. Severalmethods of polymer-solvent separation have been developed which includeflashing some solvent, for example, such as in Seebold, U.S. Pat. Nos.2,475,643, July 12, 1949, or Wallace, 3,036,057, May 22, 1962, to reacha desired polymer concentration in the solvent before separating thepolymer in solvent by further steps. Frequently, the further stepsinclude for example, further flashing and/or melt devolitilization, forexample, utilizing apparatus such as marketed by Werner and PfleidererCorporation. Where melt devolitizers are utilized, control of polymerconcentration, and flow rates in the melt devolitilizer feed areextremely important.

OBJECTS OF THE INVENTION

It is an object of this invention to simultaneously control liquidlevels in two different vessels.

It is a further object of this invention to control the flow of polymersolutions to a flash concentrator vessel.

It is a still further object of the invention to control the recycleblending of bottoms material with fresh feed to a flash concentratorvessel.

It is a further object of this invention to control the temperature andpressure of a feed stream to be fed to a flash concentrator vessel.

STATEMENT OF THE INVENTION

According to one embodiment of the invention, a method comprisesremoving liquid from a first zone; pumping a first portion of the liquidto a second zone; recycling a second portion of the liquid to the firstzone; sensing a first liquid level in the first zone and establishing afirst signal representative of said first liquid level; sensing a secondliquid level in the second zone and establishing a second signalrepresentative of said second liquid level; establishing a third signalrepresentative of a predetermined relationship between the first signaland the second signal; and manipulating the flow rate of the firstportion of liquid in response to the third signal.

In another aspect of the present invention, a method is providedcomprising flashing a first predominantly liquid stream having a firstconcentration of polymer to form an overhead stream comprisingpredominantly solvent and a predominantly liquid bottoms second streamhaving a second concentration of polymer; mixing at least a portion ofthe second stream with a third predominantly liquid stream having athird concentration of polymer to form the first predominantly liquidstream; establishing a first signal representative of the flow rate ofthe first stream; and regulating the flow rate of the second stream inresponse to the first signal.

According to another aspect of the present invention, an apparatuscomprises a first vessel; a second vessel; a first conduit meansestablishing flow communication between the first vessel and the secondvessel; a second conduit means connecting the first conduit means andthe first vessel; a means cooperating with the first vessel for sensinga liquid level in said vessel and establishing a first signalrepresentative of said liquid level; a means cooperating with the secondvessel for sensing a liquid level in said second vessel and establishinga second signal representative of said liquid level; a means forreceiving the first signal and the second signal and establishing athird signal representative of a predetermined relationship between thefirst signal and the second signal; and a means cooperating with thefirst conduit means for receiving the third signal and manipulatingfluid flow through the first conduit means in response to the thirdsignal.

According to yet another aspect of the present invention, an apparatuscomprises a pressure vessel; a first conduit means emptying into thepressure vessel; a second conduit means connected to an upper portion ofthe pressure vessel; a third conduit means connected to a lower portionof the pressure vessel and establishing a flow path between the pressurevessel and the first conduit means, said third conduit means dividingthe first conduit means into an upstream portion, and a downstreamportion between the third conduit means and the vessel; a meanscooperating with the upstream portion of the first conduit means forsensing liquid flow through the upstream portion of the first conduitmeans and establishing a signal representative of said fluid flow; and apump cooperating with the third conduit means for receiving the signalrepresentative of fluid flow through the first conduit means and causingfluid flow from the vessel and to the first conduit means in response tosaid signal.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1. FIG. 1A illustrates in schematic a portion of process flowaccording to the present invention.

FIG. 2. FIG. 1B illustrates in schematic a second portion of the processflow scheme according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A specific control system configuration is set forth in FIG. 1 for thesake of illustration. However, the invention extends to different typeof control system configurations which accomplish the purpose of theinvention. Lines designated as signal lines in the drawings areelectrical or pneumatic in this preferred embodiment. Generally, thesignals provided from any transducer are electrical in form. However,the signals provided from flow sensors will generally be pneumatic inform. Transducing of these signals is not illustrated for the sake ofsimplicity because it is well known in the art that if a flow ismeasured in pneumatic form it must be transduced to electrical form ifit is to be transmitted in electric form by a flow transducer. Also,transducing of the signals from analog form to digital form or fromdigital form to analog form is not illustrated because such transducingis also well known in the art.

The invention is also applicable to mechanical, hydraulic or othersignal means for transmitting information. In almost all control systemssome combination of electrical, pneumatic, mechanical or hydraulicsignals will be used. However, use of any other type of signaltransmission, compatible with the process and equipment in use, iswithin the scope of the invention.

Both the analog and digital controllers shown may utilize the variousmodes of control such as proportional, proportional-integral,proportional-derivative, or proportional-integral-derivative. In thepreferred embodiment, proportional-integral-derivative controllers areutilized but any controller capable of accepting two input signals andproducing a scaled output signal, representative of a comparison of thetwo input signals, is within the scope of the invention.

The scaling of an output signal by a controller is also well known incontrol system art. Essentially, the output of a controller may bescaled to represent any desired factor or variable. An example of thisis where a desired flow rate and an actual flow rate is compared by acontroller. The output could be a signal representative of a desiredchange in the flow rate of some gas necessary to make the desired andactual flows equal. On the other hand, the same output signal could bescaled to represent a percentage or could be scaled to represent atemperature change required to make the desired and actual flows equal.If the controller output can range from 0 to 10 volts, which is typical,then the output signal could be scaled so that an output signal having avoltage level of 5.0 volts corresponds to 50 percent, some specifiedflow rate, or some specified temperature, for example.

The various transducing means used to measure parameters whichcharacterize the process and the various signals generated thereby maytake a variety of forms or formats. For example, the control elements ofthe system can be implemented using electrical analog, digitalelectronic, pneumatic, hydraulic, mechanical or other similar types ofequipment or combinations of one or more such equipment types. While thepresently preferred embodiment of the invention preferably utilizes acombination of pneumatic final control elements in conjunction withelectrical analog signal handling and translation apparatus, theapparatus and method of the invention can be implemented using a varietyof specific equipment available to and understood by those skilled inthe process control art. Likewise, the format of the various signals canbe modified substantially in order to accommodate signal formatrequirements of the particular installation, safety factors, thephysical characteristics of the measuring or control instruments andother similar factors. For example, a raw flow measurement signalproduced by a differential pressure orifice flow meter would ordinarilyexhibit a generally proportional relationship to the square of theactual flow rate. Other measuring instruments might produce a signalwhich is proportional to the measured parameter, and still othertransducing means may produce a signal which bears a more complicated,but known, relationship to the measured parameter. Regardless of thesignal format or the exact relationship of the signal to the parameterwhich it represents, each signal representative of a measured processparameter or representative of a desired process value will bear arelationship to the measured parameter or desired value which permitsdesignation of a specific measured or desired value by a specific signalvalue. A signal which is representative of a process measurement ordesired process value is therefore one from which the informationregarding the measured or desired value can be readily retrievedregardless of the exact mathematical relationship between the signalunits and the measured or desired process units.

Generally, according to the invention an apparatus comprises a firstzone 2, a second zone 4, the first and second zones being connected by aconduit means 6.

The zone 2 preferably comprises two pairs 11 and 12 of vessels 8, suchas tertiary flash tanks 8. The zone 2 receives fresh feed comprisingpolymer and solvent from a reactor not shown via a conduit means 14which preferably includes a first fluid distribution means 16 whichempties into each of the first pair 11 of tanks 8 and a second fluiddistribution means 18 which empties into each of the second pair 12 oftanks 8.

The conduit means 6 preferably includes a collection means connected toeach tank 8 such as branch 21 containing a pump 22. Further, pump means,such as pump 24 cooperates with the conduit means 6 and is operable forinducing liquid flow from the zone 2 to the zone 4. Because the liquidcarried by the conduit 6 is normally of a high viscosity, pump 24 isnormally of the gear type. A suitable pump is supplied by LuwaCorporation, of Charlotte, North Carolina. A conduit means connects theconduit 6 with the zone 2. Preferably, a conduit 32 having a valve 34therein connects with the conduit 6 between the pump means 24 and thezone 4, as at connection 31. The conduit 32 carries fluid for recycle tothe vessels 8. It connects to a conduit 33 which empties into a first3-way valve 35, and a conduit 37 which connects to a second 3-way valve39. Preferably diaphragm operated valves, supplied by most any reputableinstrument equipment company are used as 3-way valve 35 and 3-way valve39. Conduits 41 and 43 define a flow path and extend between the 3-wayvalve 35 and the vessels 8 of the first pair 11. Each conduit 45 and 47defines a flow path and extends between the 3-way valve 39 and thevessels 8 of the second pair 12.

A connection 28 is associated with the conduit means 6 between theconnection 31 and the zone 4. Preferably, connection 28 is an inlinemixer with specially designed baffles and ribbons to thoroughly mix twoviscous liquids. A suitable inline mixer is supplied by Komax Systems,Inc. The contents of the conduit 6 between the connection 31 and theconnection 28 are denoted in the drawing as stream 10. A heater 36 isassociated with the conduit 6 between connection 28 and the zone 4. Avalve 38, such as a diaphragm motor valve, is disposed in the conduit 6between the heater 36 and the zone 4. The contents of the conduit 6between the connection 28 and the heater 36 are denoted on the drawingas stream 30. The contents of the conduit 6 between the heater 36 andthe valve 38 are denoted on the drawing as stream 40.

The conduit 6 empties into the zone 4, which preferably comprises aflash concentrator including a high pressure vessel 42, the conduit 6emptying thereinto via a nozzle, not shown. A conduit 50 is connected toan upper portion of the vessel 42 to withdraw overhead vapors from thevessel 42 and convey same to an overhead accumulator vessel 44, intowhich the conduit 50 empties for separation of liquids. A conduit 46having a valve 48 operably associated therewith is connected to an upperportion of the knock-out 44 for withdrawing vapors therefrom. Theconduit 46 empties into an overhead accumulator, not shown. A conduit 52having a pump 54 operably associated therewith is connected to a lowerportion of the liquid knock-out 44 for withdrawing liquid therefrom andconveying same to further processing, such as waste stripping, notshown.

The flash concentrator vessel 42 is provided with at least one conduitfor withdrawal of bottoms material. Preferably, a conduit 56 isconnected to the lower portion of the flash concentrator 42 forwithdrawal of and recycle of bottoms, and a conduit 58 is connected to alower portion of the concentrator 42 for withdrawal of bottoms materialand conveying same to further processing.

The conduit 56 establishes a flow path between the lower portion of theconcentrator 42 and the connection 28. A pump 62 operably associatedwith the conduit 56 causes fluid flow from the concentrator 42 to theconnection 28. The contents of the conduit 56 between the pump 62 andthe mixing valve 28 are designated on the drawing as stream 20.

The conduit 58 establishes a flow path between the lower portion of theconcentrator 42 and further processing apparatus such as a secondaryflash concentrator and/or a melt devolatilizer, not shown. The purposeof the melt devolatilizer is to extract the final small portion ofsolvent from the liquid stream and to convert the typically very viscousliquid into a pellet or bead form for ease of handling. The meltdevolatilizer maintains the liquid in a fluid state by means of heat andpressure and mechanically extrudes the liquid to produce the pellet. Asuitable melt devolatilizer is supplied by Werner and PfleidererCorporation of Stuttgart, West Germany. A pump 64 is operably associatedwith the conduit 58 for inducing fluid flow from the concentrator 42 tothe further downstream equipment. A heater 66 is associated with theconduit 58 between the pump 64 and the downstream equipment. Thecontents of the conduit 58 downstream of the heater 66 are designated asstream 60. A conduit 68 having a valve 72 associated therewith isconnected to the conduit 58 between the pump 64 and the heater 66 andempties back into the concentrator 42.

Operation of the apparatus is as follows.

Liquid such as a polymer solution is removed from the zone 2 via conduit6 due to the action of pumps 22 and 24. The stream is divided at theconnection 31, a first portion of the stream is passed to the secondzone 4, and a second portion of the stream is passed via conduit line 32back to the first zone 2. A level element 62 cooperating with eachvessel 8, one per vessel, senses the liquid level in its respectivevessel and establishes a signal 64 representative of said level which istransmitted to an averaging relay 68. Preferably, each signal 64 istransmitted through a level recorder 66 prior to receipt by theaveraging relay 68. A suitable averaging relay with its function toreceive an input of more than one signal, to average these signals andto transmit this average value in order to control the required streamor function can be supplied by most any major instrument equipmentmanufacturing company. The averaging relay 68 establishes a signal 72representative of the liquid level in the first zone 2. This systemtends to maintain the same level in each vessel 8.

A level element 74 associated with the vessel 42 of the second zone 4senses the liquid level in the second zone and establishes a signal 76representative of said level.

The signals 72 and 76 are received by a comparison means such as levelrecorder controller 78 which compares the signal 72 to the signal 76 andestablishes a third signal 80 representative of a predeterminedrelationship between the signal 72 and the signal 76. Preferably, thesignal 80 is a scaled difference between the signals 72 and 76 so as torepresent the required flow rate through the conduit 6 to maintain thedesired relationship between the signals 72 and 76. The purpose of thisparticular control loop is to maintain a constant level in each vessel 8as well as vessel 42. The rate at which the first portion of the liquidstream is pumped to the second zone 4 is manipulated in response to thesignal 80.

Preferably, one or the other of the signals 72 and 76 is the set pointfor the level recorder controller 78.

In the illustrated embodiment, the signal 80 is received by a flowrecorder controller 82, which also receives a signal 84 from a flowelement 86 associated with the conduit 6 downstream of the connection28, the signal 84 being representative of the rate of flow of fluidentering the zone 4. The controller 82 establishes a signal 88representative of a predetermined relationship between the signals 80and 84 such as a difference, which, after passing through a relay 90 isused to manipulate the valve 38. Valve 38 is preferably a diaphragmoperated control valve that is used to maintain a constant level invessel 42.

As valve 38 is manipulated, the fluid in the conduit 6 undergoes apressure change. A pressure element 92 is associated with the conduit 6between the connection 28 and the vessel 42 for sensing the pressure ofthe portion of the liquid being conveyed to the zone 4 and establishinga signal 94 representative of said pressure. The rate of liquid recycleto the zone 2 is manipulated in response to the signal 94 representativeof the pressure in the conduit 6. Preferably, the signal 94 is receivedby a comparison means, such as a pressure recorder controller 96 whichestablishes a signal 98 representative of a predetermined relationship,such as a difference, between the signal 94 and a set point signalrepresentative of a desired pressure in the conduit 6. Preferably, thesignal 98 is scaled so as to represent a required change in flow throughthe conduit 32 so as to maintain the desired relationship between theset point and the signal 94. Pressure in conduit 6 must be maintained ata constant value in order for flow element 86, for flow element 138 andfor connection 28 to function properly. The signal 98 is received by thevalve 34 which is manipulated in response thereto, thereby controllingthe pressure in the conduit 6.

The level in the primary flash concentrator 42 is controlled from twodirections. With less roll back to the tertiary flash tanks, theirlevels will begin to drop. The average level signal 72 sent from thetertiary flash tanks 8 will drop. Reduced roll back to the tertiaryflash tanks 8 also causes the level in the primary flash concentrator torise. The indicated level signal 76 will also rise. Thus, the set pointlevel will come down and the indicated level will rise. As the twolevels approach each other, the control actions will become smaller andsmaller, until a steady level is reached.

A similar process occurs if comparison by controller 78 shows that theindicated level 76 in the primary flash concentrator 42 is above the setpoint signal 72. The signal 80 sent to the controller 82 triggers theclosing of valve 38 slowing flow into the primary flash concentrator 42.As the valve 38 closes, pressure in the line 6 increases. The pressuretransmitter 92 relays the increased pressure to the controller 96 whichopens the valve 34 on the roll line 32 to the tertiary flash tanks 8 torelieve the pressure in the line 6. With the increased roll to thetertiary flash tanks 8, their levels will begin to rise. Higher levelsignals 64 are sent to the averaging relay 68, which sends a higher setpoint signal 72 to the controller 78. With less flow in, the indicatedlevel signal 76 and the set point signal 72 approach each other, andcontrol action decreases until a steady level is reached.

Roll back to the zone 2 is distributed among the vessels 8 by 3-wayvalves 35 and 39, which regulate flow as between the vessels 8 of a pair11 or 12. Valves can obviously be provided if desired to regulate theflow as between the pairs 11 or 12. A level controller 100 receives thesignals 64 from the first pair 11 of vessels 8 and establishes a signal102 representative of a predetermined relationship such as a differencebetween the individual signals from each vessel which is transmitted tothe valve 35. Valve 35 is a 3-way type valve that is capable ofreceiving liquid from conduit 33 and disbursing this same liquid tovessel 8 of pair 11 in a manner to maintain a constant level in eachvessel 8 in pair 11. The valve 35 is manipulated in response to thesignal 102 to increase the amount of flow to the vessel of the pair 11having the lower liquid level. Similarly, signals 64 from the vessels 8of the second pair 12 are received by a level controller 104 whichestablishes a signal 106 representative of a predetermined relationshipbetween the individual signals which is transmitted to the valve 39. Thevalve 39 is manipulated in response to the signal 106 to increase theamount of flow to the vessel of the pair 12 having the lower liquidlevel.

The flow of working fluid through the heater 36 is manipulated tocontrol the temperature of the fluid entering the zone 4. Preferably, atemperature element 108 associated with the conduit 6 between the heater36 and the vessel 42 establishes a signal 110 representative of thefluid temperature in the conduit 6 which is received by a comparisonmeans such as a temperature recorder controller 112. The temperaturerecorder controller 112 compares the signal 110 with a set point signalrepresentative of a desired fluid temperature and establishes a signal114 which is representative of a predetermined relationship between theset point signal and the signal 110. The signal 114, which is preferablyscaled so as to represent the required flow rate through the heater 36in order to maintain the desired relationship between the two signals,is received by a valve 116 positioned so as to regulate the flow ofworking fluid, such as steam, through the heater 36. The valve 116 ismanipulated in response to the signal 114, thereby controlling thetemperature of fluid entering the vessel 42. Functionally, the set pointof temperature recorder controller 112 is predetermined and set at acertain value. When the response to temperature element 108 indicatesthat the temperature of the liquid in conduit 6 between heater 36 andvessel 42 is decreasing, for example, temperature element 108 sends thesignal to temperature recorder controller 112. Since the temperature isbelow the predetermined value of the set point, signal 114 istransmitted to control valve 116, control valve 116 opens to admit moresteam to heater 36 in order to raise the temperature of the liquid inconduit 6 up to the set point value.

The flow rate of fluid overhead from the zone 4 is controlled inresponse to the pressure in the zone 4. A pressure element 118associated with the vessel 42 establishes a signal 120 representative ofthe pressure in the vessel 42 which is transmitted to a comparisonmeans, such as a pressure recorder controller 122. The pressure recordercontroller 122 establishes a signal 124 which is representative of apredetermined relationship such as a difference between the signal 120and a set point signal representative of a desired value for thepressure in the vessel 42. The signal 124 is transmitted, preferably viaa relay 126 to the valve 48, which is manipulated in response to thesignal 124. The pressure to be maintained and controlled in vessel 42 ispredetermined by process calculations and the set point on pressurerecorder controller 122 is set at this pressure value. When the pressurein vessel 42 decreases from the set point value, for example, this lowerpressure value is transmitted by pressure element 118 and by signal 120to pressure recorder controller 122. Since the pressure from signal 120is lower than the set point, this lower value is sent to relay 126 bysignal 124, which in turn closes valve 48 in order to increase thepressure in vessel 42 up to the set point value. If the pressure invessel 42 is higher than the set point on pressure recorder controller122, the process is reversed to open valve 48.

The pressure of stream 60 is controlled by manipulation of the valve 72in the roll line 68 back to the flash concentrator 42. A pressureelement 128 senses the pressure of the stream 60 and establishes asignal 130 which is representative of said pressure. The signal 130 isreceived by a comparison means, such as a pressure recorder controller132, which establishes a signal 134 representative of a predeterminedrelationship, such as a difference between the signal 130 and a setpoint signal which is representative of a desired value of the pressureof the stream 60. The valve 72 receives the signal 134 which ispreferably scaled to represent the flow rate through conduit 68 which isrequired to maintain the desired value between the set point and signal130, and is manipulated in response thereto, thereby controlling thepressure of the stream 60. The set point value at pressure recordercontroller 132 is predetermined by process calculations. If the pressurein stream 60 is lower than the set point value, for example, a signal istransmitted by pressure element 128 and signal 130 to pressure recordercontroller 132. Pressure recorder controller 132 in turn transmits asignal to valve 72 by signal 134 and calls for valve 72 to close. Valve72 will close enough to cause the pressure in stream 60 to increase toits set point value. When the pressure in stream 60 increases to a valueabove the set point value, the control loop works in reverse to openvalve 72 in order to reduce the pressure in stream 60 to the set pointof pressure recorder controller 132.

The flow rate through the conduit 56 is controlled in response to theflow rate through the conduit 6 upstream of the connection 28.Preferably, the flow rate of stream 20 is a constant multiple of theflow rate of the stream 10.

The predominantly liquid stream carried by conduit 6 downstream of theconnection 28 having a first concentration of polymer is preferablyflashed in the vessel 42 to form the overhead stream 50 and the bottomsstream 20 having a second concentration of polymer. The bottoms stream20 is conveyed to the connection 28 and mixed with the predominantlyliquid stream 10 carried by the conduit 6 immediately upstream of theconnection 20 and having a third concentration of polymer. Assuming theconcentrations of polymer at stream 10 and stream 20 are stable, theconcentration of polymer in stream 40 being fed to the vessel 42 can bemaintained relatively constant by blending streams 10 and 20 at theconnection 28 at a preselected ratio.

In accordance with this aspect of the invention, a flow element 138 isassociated with the conduit 6 between the stream 32 and the connection28. The flow element 138 establishes a signal 140 representative of theflow rate through the conduit 6 at stream 10. The signal 140 is receivedby a means for establishing a signal 142 having a predeterminedrelationship with the signal 140, such as a flow ratio recordercontroller 144. Preferably, the recorder controller 144 receives thesignal 140 and multiplies it by a set point signal to produce the signal142. The set point signal can be selected from a wide range of values.Preferably, however, a set point signal having a value within the rangeof from about 0.10 to about 1.0, more preferably, within the range offrom about 0.15 to about 0.75 is utilized.

The signal 142 is received by a suitable comparison means, such as aflow recorder controller 146, which establishes a signal 148representative of a predetermined relationship, such as a differencebetween the signal 142 and a signal 150. Preferably, the signal 148 issuitably scaled to represent a desired speed of a motor 160 to maintaina desired relationship between the signals 142 and 150. The ratio of thetotal flow of stream 10 and of the total flow of stream 20 is maintainedat a constant value as determined by process calculations. Normally thetotal flow of stream 10 will remain constant, however if stream 10 doesvary, the flow of stream 20 will change in order to maintain a constantratio between stream 10 and stream 20. In practice, the set point offlow ratio recorder controller 144 is predetermined. If the flow ofstream 10 increases, this signal is transmitted by flow element 138 andsignal 140 to flow ratio recorder controller 144. The output signal 142is sent from flow ratio recorder controller 144 to flow recordercontroller 146. Since signal 142 becomes higher than the flow set pointon flow recorder controller 146, signal 148 is transmitted to speedcontroller 154, which in turn transmits signal 156 to motor 160. Sincemotor 160 is of the variable speed type, the signal 156 calls for themotor 160 to increase in speed, which in turn will increase the flow ofliquid through pump 62 to the point where the ratio of stream 10 andstream 20 is of the proper value. When the flow of stream 10 decreases,the control loop will reverse the procedure to reduce the speed of pump62 to maintain a constant ratio between stream 10 and stream 20. Thesignal 150 is representative of the rate of flow through the conduit 56,and is established, for example by a speed element 152 coupled to pump62, for example, where pump 62 is of the positive displacement type.

The signal 148 is received by a speed controller 154 which produces asuitable signal 156 which is relayed to a motor 160 coupled to the pump62, thereby controlling the speed of the pump 62 and thus the flow rateof stream 20, in response to the flow rate on the stream 10.

The invention is further illustrated by the following Table, showingcalculated balances for a preferred embodiment, with reference to FIG.2.

                                      TABLE I                                     __________________________________________________________________________    STREAM   10   20   30   40   50   60                                          __________________________________________________________________________    Component.sup.1                                                               Ethylene 8856 1080 9936 9936 5285 3571                                        Polymer  360,000                                                                            109,968                                                                            469,968                                                                            469,968   360,000                                     Solvent.sup.2                                                                          2,211,408                                                                          380,280                                                                            2,480,640                                                                          2,480,640                                                                          1,318,560                                                                          892,848                                     Wt. % Polyer                                                                           13.95                                                                              28.92                                                                              15.87                                                                              15.87     28.65                                       Temperature.sup.3                                                                      266  285  268  390  285  390                                         Pressure.sup.4                                                                         370  370  340  300  44   300                                         __________________________________________________________________________     .sup.1 Pounds Per Day                                                         .sup.2 (wt %) 66% cyclohexane, 23% N--hexane, 11% dimethylpentane             .sup.3 °F.                                                             .sup.4 Pounds Per Square Inch, guage                                     

What is claimed is:
 1. A method comprising(a) flashing a predominantlyliquid stream having a first concentration of polymer in a flash zone toform an overhead stream comprising predominantly solvent; (b) pumping apredominantly liquid bottoms second stream having a second concentrationof polymer from the flash zone; (c) mixing the second stream with athird predominantly liquid stream having a third concentration ofpolymer to form the first predominantly liquid stream; (d) establishinga first signal representative of the flow rate of the third stream; and(e) regulating the flow rate of the second stream in response to saidfirst signal.
 2. A method as in claim 1 wherein the pumping of thesecond stream from the flash zone is controlled in response to the firstsignal.
 3. A process as in claim 2 further comprising receiving thefirst signal by a suitable means and establishing a second signal inresponse to the first signal, said second signal leaving a predeterminedrelationship with respect to the first signal and being representativeof a desired rate at which the second stream is to be mixed with thethird stream.
 4. A process as in claim 3 wherein the second signal isrepresentative of a flow rate between about 0.1 and 1.0 of the flow rateof the third stream.
 5. A process as in claim 4 wherein the secondsignal is representative of a flow rate between about 0.15 and about0.75 of the flow rate of the first stream.
 6. A process as in claim 4wherein the first stream comprises within the range of from about 12 toabout 20 weight percent polyethylene in solvent, wherein the secondstream comprises within the range of from about 20 to about 40 weightpercent polyethylene in solvent, and wherein the third stream comprisesfrom about 10 to about 18 weight percent polyethylene in solvent, thepercentages based total weight of polyethylene and solvent.